R6(a): de-stage code, docs, READMEs; split stage6_test

Mechanical, behaviour-preserving removal of Stage N / TODO-N / phase (RN)
references from comments, doc-comments, service READMEs, the current-state docs
(ARCHITECTURE, FUNCTIONAL+_ru, TESTING, UI_DESIGN), config-file comments, and the
.fbs/.proto schema comments. PLAN.md / PRERELEASE.md / CLAUDE.md keep the stage
history.

- Rename the only stage-named identifiers: registerStage8 -> registerSocialOps,
  registerStage11 -> registerLinkOps (gateway transcode).
- Split stage6_test.go: TestEmailLoginFlow -> email_test.go,
  TestGuestAutoMatchLeavesNoStats (+ provisionGuest) -> account_test.go.
- Regenerated proto bindings (push.pb.go, telegram_grpc.pb.go) from the de-staged
  .proto comments; FB Go/TS bindings unchanged (flatc strips schema comments).

go build/vet/gofmt clean across modules; integration typecheck and pnpm check green.

backend/internal/inttest/admin_test.go

@@ -169,7 +169,7 @@ func TestConsoleServesAndGuardsCSRF(t *testing.T) {
 }
 
 // TestConsoleGameDetailRobotSchedule checks the admin game card surfaces a robot seat's
-// play-to-win intent and, while it is the robot's turn, its next-move ETA (Stage 17).
+// play-to-win intent and, while it is the robot's turn, its next-move ETA.
 func TestConsoleGameDetailRobotSchedule(t *testing.T) {
 	ctx := context.Background()
 	svc := newGameService()
@@ -207,7 +207,7 @@ func TestConsoleGameDetailRobotSchedule(t *testing.T) {
 	}
 }
 
-// TestConsoleThrottledViewAndFlagClear drives the R3 rate-limit surface end to
+// TestConsoleThrottledViewAndFlagClear drives the rate-limit surface end to
 // end against real stores: a gateway report past the threshold auto-flags the
 // account, the throttled view shows the episode and the flagged account, the
 // user card carries the marker, and the operator clear (a same-origin POST)